1. Field of the Invention
The present invention relates to ultrasonic-actuator driving apparatuses and ultrasonic-actuator driving methods. More particularly, the present invention relates to an ultrasonic-actuator driving apparatus and an ultrasonic-actuator driving method in which applying frequency signals to an ultrasonic transducer in an ultrasonic actuator friction-drives a driven body that is in contact with the ultrasonic transducer.
2. Description of the Related Art
Ultrasonic motors (also referred to as ultrasonic actuators) have drawn attention in recent years as new motors which can be used in place of electromagnetic motors. The ultrasonic actuators have the following advantages, compared with known electromagnetic motors.    (1) Low speed and high torque yielded without using gears    (2) High maintaining power of driving force    (3) Long stroke and high resolution    (4) Quiet    (5) No magnetic noise produced and no noise influence
A driving apparatus ordinarily drives each of the ultrasonic actuators having the above advantages.
Known arts involving such a driving apparatus for an ultrasonic actuator include a controlling apparatus for a vibration actuator disclosed in U.S. Pat. No. 6,031,316.
The controlling apparatus for a vibration actuator (also referred to as an ultrasonic actuator) disclosed in the above publication evaluates a real moving state of a driven body (moving body) for driving the ultrasonic actuator in order to overcome the problem in that the position control of the ultrasonic actuator is unstable.
In other words, the controlling apparatus for an ultrasonic actuator has a moving-state detecting unit for outputting signals corresponding to the absolute motion of the driven body with respect to the ultrasonic actuator. The signals output from the moving-state detecting unit are sampled at a timing being synchronized with a multiple-fold frequency of the vibration frequency of the ultrasonic actuator to evaluate the moving state based on a value in accordance with the average value of the sampled signals.
Specifically, as shown in FIG. 1 of the drawings of the U.S. Pat. No. 6,031,316, signals detected by a rotary encoder 6, which corresponds to the moving-state detecting unit, are supplied to a counter 9, which counts the signals. Edges of signals (S-phase pulse signals) detected in a vibration detecting element provided in an electro-mechanical energy conversion element are detected by a pulse-edge detecting circuit and latch signals are supplied to a latch for latching the counter values. In this case, the counter values are latched at rising edges and falling edges of the S-phase pulse signals to be supplied to an adder for addition. The values added in the adder are used to calculate an average value in a CPU. The moving state is evaluated by using the average value as a central value (a real value) of the counter values.